A growing number of shooting enthusiasts are creating legal trusts to acquire fully automatic guns, silencers or other firearms whose sale is restricted by federal law — a mechanism that bypasses the need to obtain law enforcement approval or even criminal background checks.

The trusts, called gun trusts, are intended to allow owners of the firearms to share them legally with family members and to pass them down responsibly. They have gained in popularity, gun owners say, in part because they may offer protection from future legislation intended to prohibit the possession or sale of the firearms.

But because of a loophole in federal regulations, buying restricted firearms through a trust also exempts the trust’s members from requirements that apply to individual buyers, including being fingerprinted, obtaining the approval of a chief local law enforcement officer and undergoing a background check.

Lawyers who handle the trusts and gun owners who have used them say that the majority of customers who buy restricted firearms through trusts do not do so to avoid such requirements. And most gun dealers continue to require background checks for the representative of the trust who picks up the firearm. But not all do. Christopher Dorner, the former Los Angeles police officer who embarked on a week-long assault on law enforcement officers this month that ended with his death on Feb. 12, said in a rambling 11,000-word manifesto that he had used a gun trust to buy silencers and a short-barreled rifle from a gun store in Nevada without a background check.

Referring to a computer program available from the personal finance software company Quicken, Dorner wrote, “I was able to use a trust account that I created on quicken will maker and a $10 notary charge at a mailbox etc. to obtain them legally.” Dorner was not a felon and probably would have passed a background check had he received one.

Mike Campbell, a spokesman for the Bureau of Alcohol, Tobacco, Firearms and Explosives, which enforces firearms regulations, said that applications filed with the ATF for transfers of restricted firearms to trusts or corporations have more than doubled in the last four years, to more than 39,000 in 2012 from about 15,000 in 2008. He said the increase was largely attributable to the growth in the number of trusts.

Campbell confirmed that under current regulations, background checks were not required for the buying of restricted firearms through trusts. He added that the agency was aware of the loophole and was reviewing changes to close it.

The cost of setting up a trust can vary from a small sum for an online form to $100 to $2,500 in lawyers’ fees, depending on location and the type of trust.

Gun owners also turn to trusts, other lawyers who handle them said, because in many jurisdictions, law enforcement officials refuse to sign off on the purchase of restricted firearms, making it difficult or impossible for enthusiasts to buy them as individuals.

Brian Reynolds, a lawyer in Denver, said he had prepared several gun trusts, mostly for people who wanted to buy silencers for long-range target shooting. But in many parts of Colorado, he said, sheriffs and police chiefs will not approve such purchases.

“By having a trust, you bypass the need to get that authorization,” Reynolds said.

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